fix require

Author: gares

HB/structures.v

@@ -1,5 +1,5 @@
 (* Support constants, to be kept in sync with shim/structures.v *)
-From Coq Require Import ssreflect ssrfun.
+From Corelib Require Import ssreflect ssrfun.
 
 Variant error_msg := NoMsg | IsNotCanonicallyA (x : Type).
 Definition unify T1 T2 (t1 : T1) (t2 : T2) (s : error_msg) :=